1. Field of the Invention
The present invention relates to a light-receiving element of an image sensor used for such an image-reading system as a digital camera, an image scanner, a facsimile, a copying machine, etc., as well as a photoelectric conversion device comprising the light-receiving element, specifically to the structure of a light-receiving element suitable for a photoelectric conversion device such as a close contact type image sensor, which is provided with a comparatively large light-receiving element having the opening portion of a pixel with several tens microns or more in size.
2. Related Background Art
In recent years, CCD image sensors, non-CCD image sensors such as CMOS image sensors, etc. have been positively developed as photoelectric conversion devices.
Generally, a photodiode comprising a PN junction of a semiconductor is used for light-receiving elements of these photoelectric conversion devices.
Prior Art (1)
For example, as disclosed by Japanese Patent Application Laid-Open No. 55-154784, there is proposed a structure of a light-receiving element in which the surface of a substrate where no PN junction is formed has a region having the same conductivity type as that of the substrate and a larger impurity concentration than that of the substrate, thereby reducing the dark current to be generated on the surface of the substrate.
FIGS. 29A and 29B show a structure of a conventional light-receiving element. Numeral 201 indicates an n-type semiconductor substrate; 202: a p-type semiconductor layer; 203: an n-type semiconductor layer having an impurity concentration of 5×1015 cm−3 to 10×1015 cm−3 and a thickness of 0.2 μm to 0.3 μm; 205: a thermal oxide film; 208: an n+ channel stopper; 209: an anti-reflection coating film consisting of nitride; 215 and 216: aluminum electrodes; 228: an n+-type semiconductor layer; and 238: a surface electrode. Symbol DL indicates a depletion layer; and DLS: a surface side portion of the depletion layer.
The anode of a photodiode is formed only with a p-type semiconductor layer 202. When the impurity concentration is lowered, therefore, the property of the ohmic contact with the electrode 215 is degraded. On the contrary, when the impurity concentration is increased, the depletion layer DL is not extended into the semiconductor layer 202.
Prior Art (2)
As disclosed by Japanese Patent Application Laid-Open No. 61-264758, there is proposed a light-receiving element in which the junction capacitance formed by the PN junction is reduced, as the light-receiving element for a one-dimensional photoelectric conversion device.
FIG. 30 shows a top view of a conventional photoelectric conversion device such as a CCD image sensor. Numeral 301 indicates a p-type substrate and numeral 302 indicates an n+-type accumulating portion. A portion surrounded by the n+-type accumulating portion 302 on the p-type substrate 301 is a p-type photoelectric conversion region functioning as a pixel. Symbol PG indicates a photo-gate; SG: a shift-gate; and SR: a CCD shift-register.
In this structure, the PN junction area is reduced, but the PN junction periphery length is increased. It is therefore impossible to sufficiently reduce the capacitance of the PN junction, whereby the sensitivity cannot be increased so much.
Prior Art (3)
For example, as disclosed by Japanese Patent Application Laid-Open No. 1-303752, there is proposed a structure of a photosensitive portion used for a close contact type image sensor, which is intended to reduce the dark current to be caused by the scribe at the end of the chip in the structure of the photosensitive portion.
FIG. 31 shows a cross-sectional view of a light-receiving element of prior art. In FIG. 31, numeral 301 indicates a P-type semiconductor region; 302: an N-type semiconductor region; 303: a P-type shallow channel stop layer; 305: a field oxide film; 306: a P-type substrate; 308: a P-type channel stop layer; 309: an interlayered insulating film; 317: a light-shielding film for forming an opening portion (OP). A depletion layer (DL) is extended into the P-type semiconductor region 301, whereby electrons of the generated photocamers (PC) are collected in the N-type semiconductor region 302 by an internal magnetic field.
Prior Art (4)
For example, as disclosed by Japanese Patent Application Laid-Open No. 64-14958, a photodiode having a cross-sectional structure of N-type substrate/P-type region/N-type region/P-type region is generally used as a light-receiving element of a CCD image sensor.
FIG. 32 shows a cross-sectional view of a light-receiving element of prior art. In FIG. 32, numeral 406 indicates an N-type substrate; 401: a P-type semiconductor region; 402: an N-type semiconductor region; 403: a shallow P-type semiconductor layer; 408: a P+-type channel stop layer; 409: an insulating film; 415: an electrode consisting of polysilicon; and 420: an N-type region of a CCD register.
Prior Art (5)
On the other hand, a photoelectric conversion device employing a light-receiving element is proposed, for example, in Japanese Patent Application Laid-Open No. 9-205588, which uses a photodiode as a light-receiving element and reads the electric charges of the light-receiving element at a time with use of a source follower amplifier by providing this light-receiving element with an electrode and connecting it to the gate electrode of an MOS transistor.
However, when a light-receiving element is employed for an amplifying type photoelectric conversion device which accumulates photo-generated carriers and reads out a signal voltage from a PN photodiode by using charge-voltage conversion means, the sensitivity of the photoelectric conversion device may be degraded in some cases.
For such an amplifying type photoelectric conversion device, the light output is represented by the following formula (1):Vp=Qp/Cs  (1)wherein Qp is the quantity of a charge accumulated in the PN photodiode and Cs is a capacitance of the photodiode.
The capacitance Cs of this photodiode can be represented by the formula (2) as shown below, for example, for an amplifying type photoelectric conversion device having a pixel in which an MOS source follower or a reset MOS transistor is connected to a photodiode:Cs=Cpd+Ca  (2)Wherein Cpd is a PN junction capacitance of the PN photodiode itself including a light-receiving portion, Ca is the other capacitance of portions connected to the photodiode, and in the above case the other capacitance includes the gate capacitance of the MOS transistor constituting the MOS source follower, the capacitance of the junction between the source and the well of the reset MOS transistor, the overlapping capacitance of the source and the gate, the wiring capacitance, and the like.
Consequently, to realize high sensitivity, it is indispensable that the photo-generated carriers are effectively accumulated and the capacitance of the photodiode for accumulating the carriers is reduced as much as possible.
On the other hand, when light is made incident in the photodiode, electric charges generate in the photodiode and electric charges in and around the depletion layer formed due to a PN junction surface in the semiconductor substrate gather at an anode or a cathode. In this case, when an electrode is attached to this anode or cathode, the electric charges can be taken out as electric signals.
Prior Art (6)
FIG. 33 is a cross-sectional view of a light-receiving element of prior art provided with an electrode. In FIG. 33, numeral 701 indicates a first semiconductor region, and numeral 702 indicates a second semiconductor region to be used as an anode. The conductivity types of those regions are N-type and P-type, respectively. In addition, symbol DL is a depletion layer formed by a PN junction between the first semiconductor region 701 and the second conductor region 702. Although not illustrated in FIG. 33, a reverse bias is applied to between the first semiconductor region 701 and the second semiconductor region 702. In addition, numeral 715 indicates an electrode, which is connected to the second semiconductor region 702 through a contact hole CH of the insulating film 709.
The electrode 715 is composed of a metal, for example, Al as a main component. The electrode 715 is connected to an electrode region formed on the major surface of the semiconductor substrate through a contact hole CH of the insulating film covering the surface of the photodiode. Generally, such light-receiving element is composed by connecting a conductive material such as Al to a semiconductor region so as to obtain photosignals generated by photocarriers photoelectric-converted in the semiconductor region.
For example, when a general RIE (reactive ion etching) method is employed to form this electrode, then over-etching is usually conducted so as to remove unnecessary portions. In this over-etching, however, some ions accelerated by an electrical field pass through the insulating film 709 and reach the major surface of the semiconductor substrate and then damage the vicinity of the interface between the semiconductor and the insulating film, thereby resulting in generation of crystal defects in some cases.
Crystal defects may also generate due to the plasma ashing of a photoresist, etc. even in a step after the electrode is formed, just like in the above case.
In the case of a general light-receiving element, a PN junction exists around a semiconductor region formed on the major surface of the semiconductor substrate to which an electrode is connected and the junction surface reaches the vicinity of the interface between the major surface of the semiconductor substrate and the insulating film in many cases.
Consequently, when an electrode is formed at an inner portion from the junction surface reaching the major surface of the semiconductor substrate, crystal defects due to etching damage generate in the vicinity of the junction surface, and the crystal defects become centers for causing carriers to be generated. The crystal defects generated at a portion of the depletion layer cause a dark current to be generated.
The generated dark current as described above also becomes a factor for causing the dark current to be varied, since the quantity of the crystal defects generated in the vicinity of the interface or the quantity of crystal defects themselves is changed by misalignment of a mask in formation of electrodes, etc. and etching conditions.